The invention relates to improvements in apparatus for trimming, clipping or slitting lateral marginal portions of sheets which consist of paper or other flexible sheet material More particularly, the invention relates to improvements in trimming, clipping or slitting apparatus (hereinafter called trimming apparatus) which can be utilized with advantage to remove surplus material from lateral marginal portions of partially overlapping sheets which are transported in the form of a so-called scalloped or imbricated stream.
Swiss Pat. No. 650 967 discloses a trimming apparatus wherein conveyors are employed to transport a stream of partly overlapping sheets in a predetermined direction along an elongated path past two pairs of cooperating knives and counterknives, one pair at each side of the path. In order to ensure the making of clean cuts at the front and trailing ends as well as at other parts of successive sheets, irrespective of the thickness of individual sheets (the term "sheets" is intended to embrace individual sheets, sets of fully overlapping individual sheets, folded individual sheets and folded sets of two or more sheets), the cutting edges of blades forming part of rotary knives in the patented apparatus are oriented to make with the transporting plane for the sheets an angle of 0.degree.-15.degree.. In addition, when a blade is in a position of maximum overlap with the associated counterknife, it makes with the aforementioned plane an angle of 15.degree. to 45.degree..
It has been found that, in actual practice, the above undertakings do not ensure the making of a clean cut all the way from the leader to the trailing end of each sheet. Thus, if the stream is assembled of partly overlapping groups of several sheets, e.g., signatures containing a relatively large number of printed pages, the blades of the knives which are located at a level above the stream are likely to tear the sheets substantially midway between the leaders and the trailing ends of the sheets. Attempts to avoid such tearing include changing the angle between the blades and the plane of the stream at the instant of maximum overlap of blades and the respective counterknives, changing the RPM of the knives and/or changing the shape of the counterknives. None of these undertakings can ensure reliable prevention of tearing of the sheets; this holds true for trimming apparatus of the type disclosed in the Swiss patent as well as for other types of trimming apparatus.